DNA damage produced by 5-azadeoxycytidine in mouse leukemia L1210 cells was studied using the alkaline elution technique. DNA that was synthesized during the drug exposure period, and which presumably contained incorporated azacytosine residues, was found to contain alkali-labile lesions, suggestive of base-free sites. The hypothesis is being pursued that these base-free sites are a consequence of loss of azacytosine residues from the DNA, either by spontaneous decomposition or by the action of a glycosylase type of DNA repair enzyme.